1. Field of the Invention
The present invention relates to an in-jet cartridge, including a head cartridge and an ink tank which are detachably attached to each other, which is used in an ink-jet recording apparatus, and also relates to an ink-jet recording apparatus having such an ink-jet cartridge. The present invention particularly relates to an ink-jet cartridge and an ink-jet recording apparatus in which a change in pressure of the ink in the inside of ink discharge nozzles in the head cartridge is reduced.
2. Prior Art
Heretofore, as an ink-jet cartridge used in an ink-jet recording apparatus, there is known an ink-jet cartridge of the type constituted by a head cartridge having an ink-jet recording head and a tank holder, and an ink tank capable of being attached to and detached from the tank holder.
From the point of view of reduction in cost, this type ink-jet cartridge is more advantageous than an ink-jet cartridge including a head cartridge and an ink tank which are prohibited from being detached from each other, because this type ink-jet cartridge is such that only the ink tank can be exchanged at the time of consumption of ink. In an ink-jet printer using such an ink tank, there are some cases where the liquid surface of ink in the ink tank is higher than the surface of the ink discharge nozzles of the recording head. In these cases, water head difference pressure caused by the difference in level of the liquid surface of ink is imposed on ink in the inside of the ink discharge nozzles of the recording head, so that ink flows out of the nozzles. To prevent the outflow of ink, a negative pressure generating means is required for applying a negative pressure into the ink tank.
As a method for storing ink in an ink tank, there is generally and widely used a method incorporating capillary force (liquid keeping force in accordance with a capillary phenomenon) of a soft porous ink keeper such as sponge disposed in the ink tank to thereby hold ink, as described in Japanese Patent Unexamined Publication Nos. Sho-63-87242 and Hei-2-34349.
In the case where an ink tank having such a porous ink keeper put therein is used, there is no generation of capillary force if the ink tank is entirely filled with ink. As a result, the pressure in the inside of the ink discharge nozzles becomes equal to the atmospheric pressure to thereby bring about leakage of ink out of the nozzles. Therefore, the ink tank is conventionally used so as to be filled with ink by about 70% with respect to the whole volume of the ink tank, because it is impossible to entirely fill the ink keeper with ink.
In this case, ink is kept by the capillary force of a portion where the ink keeper is not filled with ink, so that the pressure of ink in the inside of the ink discharge nozzles is kept in at a predetermined negative value (for example, -30 mmH.sub.2 O).
In the case where the above-mentioned ink keeper is used, when the volume of the ink keeper portion not filled with ink increases the consumption of ink, the capillary force increases so that the ink with which the ink keeper is filled partly remains in the ink keeper without being consumed. Accordingly, there arises a problem in that the percentage of filling ink used is only about 80%. Therefore, not only it is difficult to reduce the size of the cartridge, because the efficiency of use of the volume of the ink tank is in the range of only from about 50% to about 60%, but reduction in running cost is inhibited because the efficiency of use of ink is inferior.
Further, as the residual quantity of ink decreases, the water head pressure caused by the difference in the level of the ink tank decreases and the above-mentioned capillary force increases. Accordingly, there occurs an increase of the negative pressure in the inside of the ink discharge nozzles. The increase of the negative pressure causes instability of the characteristic of jetting ink from the recording head. Therefore, a method in which a pressure adjusting valve is provided between the ink tank and the recording head has been proposed as described in Japanese Patent Unexamined Publication No. Sho-62-231759. The proposed method cannot be adapted for practical use because the pressure adjusting valve is apt to be broken down due to the injury of the valve, the blocking of the valve, etc., and because leakage of ink out of the ink tank occurs when the valve is broken down.
Further, as an idea for suppressing the increase of the negative pressure of ink in the ink filling type ink tank using a porous member in its inside, there has been published "An Optimization Study Of A Drop-On Demand Ink Jet Print Head Cartridge" (Michael F. Baumer et al., issued by IS&T, Jun. 10, 1991, IS&T's Seventh International Congress on Advances in Non-impact Printing Technologies, Volume 2, pp. 96-106). In this literature, melamine foam is used as the porous member, and there is proposed an ink tank having such a shape that the compression rate of the melamine foam increases in accordance with the height of the ink tank. That is, the ink tank is formed so that its upper portion has a smaller sectional area but the sectional area of the ink tank increases in the order from its upper portion to its lower portion. Further, the ink tank is filled with the melamine foam so that the compression rate of the melamine foam becomes higher at its upper portion.
In this type ink tank, the change of the water head pressure caused by the difference in level of the ink tank can be canceled by changing the compression rate of the melamine foam. The increase of the compression rate of the melamine foam, however, brings about a large final residual quantity of ink. Accordingly, the efficiency of use of ink is still small.
To reduce the change of the water head pressure, it is thought of that the height of the ink tank maybe reduced. In the case where the height of the ink tank is merely reduced, however, the horizontal sectional area of the lower portion of the ink tank must be large if the internal volume of the ink tank is not changed. As a result, an ink tank supporting portion of the head cartridge becomes large in its plane shape. Further, the ink tank supporting portion of the head cartridge is generally supported by the bottom portion of the head carriage of the ink-jet recording apparatus. In this case, the bottom surface of the head carriage for supporting the head cartridge also becomes large in its plane shape. Furthermore, the fact that the height of the ink tank is reduced means that the ink tank is disposed in proximity to the bottom surface of the head carriage. As described above, when the plane shape of the bottom surface of the head carriage increases, a large space in a horizontal surface is spent in the periphery of the lower portion of the head carriage.
In the ink-jet recording apparatus, however, various constituent elements (for example, head carriage movement control members such as a belt for driving the head carriage, a slit train of a predetermined pitch for detecting the position of the head carriage through a photosensor, etc., conveyance members for conveying a recording sheet, and the like) must be disposed in the periphery of the lower portion of the head cartridge. In this case, when the height of the ink tank is merely reduced, the plane shape of the lower portion of the head carriage for supporting the ink tank increases. As a result, it is difficult to secure a space for arrangement of the various constituent elements.